This invention relates to a novel process for preparing metal oxide pigment dispersions. More particularly, this invention relates to pigment dispersions that are well-suited to color cosmetics.
Various cosmetic products, such as loose or compact powders, make-up foundations, blushes, eye shadows and lipsticks are colored using inorganic pigments dispersed in a carrier. Examples of these inorganic pigments are iron oxides, zinc oxide, talc, titanium dioxide, chromium hydroxide, and chromium oxide. The formed cosmetic products may be oil-in-water emulsions, water-in-oil emulsions or anhydrous compositions.
In general, cosmetics must (1) feel smooth and apply uniformly, (2) be easy to remove, (3) be hard to smudge and (4) be non-oily. Water or aqueous based cosmetics satisfy most of these requirements, usually in the form of oil-in-water emulsions. Oil-in-water emulsions are desirable because they have a pleasing skin-feel and texture. Moreover, they do not have the greasy feel of water-in-oil emulsions or the rough, dry, powdery feel of anhydrous cosmetic compositions.
Despite the desirable properties of oil-in-water emulsions, it is difficult to prepare these emulsions while maintaining even dispersion of inorganic pigments within the emulsions. This is because inorganic pigments, especially metal oxide pigments, tend to agglomerate. Specifically, the pigment particles attract to one another and form a colloid or enlarged clump of pigment when added directly to oil-in-water dispersions. If the pigments are not evenly dispersed, aesthetically unpleasing dark spots or swirls may appear in the final cosmetic. Further, the uneven dispersions or agglomeration of pigment particles creates an abrasive feel on the skin.
A variety of methods are used to enhance dispersion and prevent agglomeration of inorganic pigments. In all of these methods, the pigment particles are treated and directly added to the oil-in-water emulsion or the water phase of the oil-in-water emulsion to provide coloring. One method disclosed in U.S. Pat. No. 5,260,052 to Peters et al, coats inorganic pigments with polymers to promote even dispersion in a water base. Another method in U.S. Pat. No. 6,004,567 to Marchi-Lemann uses nano-pigment particles mixed with fillers and dispersed in a water base. Yet another method directly coats metal oxide pigments with a hydrophilic surfactant to increase its water dispersibility when added directly to an oil-in-water emulsion.
The above methods produce acceptable results for some inorganic pigments, but typically fail to evenly disperse metal oxide pigments in the emulsion. This problem is particularly acute when using black iron-oxide pigments. Due to their magnetic nature, iron-oxide pigments agglomerate even when they are treated with the methods above. Thus to keep the pigment particles evenly dispersed in the oil-in-water emulsion, high-speed shearing mixers are used. Even when such techniques are implemented, re-agglomeration may occur in holding tanks before the finished product is poured into retail sale packaging. To prevent this, many manufacturers add a second high-speed mixer to the holding tank. In some cases, however, a high-speed mixer over-agitates, whips or froths the product before it is poured into retail containers. Resulting froth or air bubbles can cause color changes, discoloration, air pockets and/or pigment particle agglomeration in the final product.